This invention relates generally to the field of photographic cameras, and more particularly to an improved range finder type focusing screen widely used in reflex type cameras, particularly single lens reflex types which have become increasingly popular in recent years.
Traditionally, such screens are formed of rectangular panels of glass, a lower surface of which is ground to permit the formation of a real image which may be focused. To facilitate focusing it is known to provide at a centrally disposed area, a pair of prisms with the angularly disposed planes thereof symmetrically positioned at opposite angles relative to the focusing plane to create a so-called split image which is unified when the image is in focus upon the ground glass. This construction, while useful, has a disadvantage in that depending upon the angle of the exposed prism surfaces relative to the ground glass surface, difficulty is encountered in focusing other than lenses of normal focal length, particularly when used at other than the widest aperture. In an attempt to overcome this difficulty, the prisms are sometimes supplemented by an annular area containing small microprisms which distort the viewed image when in other than correct focus. The microprism area supplements the split image prism and replaces it when using telephoto and wide angle lenses, where, because of the particular angle of incidence upon the prisms, one or the other of the prisms "blacks out". It is also known to provide a so-called Fresnel rings over selective areas of the finder to provide more even illumination.
In practice, the above constructions are not ideal. The split image prisms either with or without Fresnel type surfaces will, as mentioned above, often black out, rendering them inoperative. The microprism area does not "black out" but tends to offer very little warning to the user prior to appearance of correct focus, resulting in considerable overshooting and hunting until precise focus is obtained. It is unfortunate that the microprism area is used with lenses of relatively longer focal length, in which the depth of field is relatively short.